Refactor: extract lib.rs into focused sub-modules, add --output-dir CLI option

- Split monolithic src/lib.rs (~640 lines) into 7 sub-modules:
  types.rs, context.rs, user_data.rs, translate.rs, http.rs,
  csv_util.rs, test_utils.rs
- Add configurable --output-dir CLI option for CSV output files
- Implement atomic file writes (write-to-temp + rename) for all CSVs
- Change compute_car_counts() to return CarCounts directly (not Result)
- Add test-utils feature gate for shared test helpers
- Add HTTP mock tests with mockito (6 tests)
- Add proptest tests for compute_car_counts (3 property tests)
- Update integration tests to use shared test utilities
- Update documentation (API.md, ARCHITECTURE.md, DATA_STRUCTURES.md)

tests/integration_tests.rs

@@ -11,35 +11,7 @@ use top_drives::{
     compare_cars, compute_car_counts, escape_csv_field, load_my_cars_data,
     translate_track_name, translate_track_type,
 };
-
-// -- Helpers --
-
-fn make_record(car_rid: &str, rq: u8, time: f64, guid: &str) -> BestCarRecord {
-    BestCarRecord {
-        car_rid: car_rid.to_string(),
-        brand: "TestBrand".to_string(),
-        only_name: "TestModel".to_string(),
-        year: 2020,
-        rq,
-        config: "111".to_string(),
-        time,
-        owned: false,
-        prize: false,
-        guid: guid.to_string(),
-    }
-}
-
-fn make_car(rid: &str, rq: u8, guid: &str) -> Car {
-    Car {
-        rid: rid.to_string(),
-        brand: "TestBrand".to_string(),
-        only_name: "TestModel".to_string(),
-        year: 2020,
-        rq,
-        prize: false,
-        guid: guid.to_string(),
-    }
-}
+use top_drives::test_utils::{make_record, make_car};
 
 // =======================================================================
 // Integration: AnalysisContext → CarCounts pipeline
@@ -49,7 +21,7 @@ fn make_car(rid: &str, rq: u8, guid: &str) -> Car {
 fn test_full_pipeline_empty_input() {
     let ctx = AnalysisContext::new();
     let car_list: Vec<Car> = vec![];
-    let counts = compute_car_counts(&ctx, car_list).unwrap();
+    let counts = compute_car_counts(&ctx, car_list);
 
     assert!(counts.by_rid.is_empty());
     assert!(counts.guid_to_rid.is_empty());
@@ -70,7 +42,7 @@ fn test_full_pipeline_single_car_multiple_tracks() {
     }
 
     let car = make_car("Ferrari_F40", 95, "guid_f40");
-    let counts = compute_car_counts(&ctx, vec![car]).unwrap();
+    let counts = compute_car_counts(&ctx, vec![car]);
 
     assert_eq!(counts.by_rid.len(), 1);
     let (rec, count) = counts.by_rid.get("Ferrari_F40").unwrap();
@@ -124,7 +96,7 @@ fn test_full_pipeline_multiple_cars_different_rq() {
         make_car("Car_C", 100, "guid_c"),
     ];
 
-    let counts = compute_car_counts(&ctx, car_list).unwrap();
+    let counts = compute_car_counts(&ctx, car_list);
 
     assert_eq!(counts.by_rid.len(), 3);
 
@@ -265,7 +237,7 @@ fn test_ownership_filtering_in_counts() {
         make_car("Unowned_Car", 90, "guid_unowned"),
     ];
 
-    let counts = compute_car_counts(&ctx, car_list).unwrap();
+    let counts = compute_car_counts(&ctx, car_list);
 
     // Both should be in by_rid
     assert_eq!(counts.by_rid.len(), 2);